A Separation by Implanted Oxygen (“SIMOX”) wafer and a bonded wafer are known as typical examples of an SOI substrate which has a single crystal silicon layer formed on an insulating material such as silicon dioxide. The SIMOX wafer is an SOI substrate obtained by: introducing oxygen ions inside a single crystal silicon substrate through the ion implantation of oxygen ions; having the oxygen ions react with silicon atoms through a subsequent annealing process, and thus forming a Buried Oxide layer (hereinafter called a BOX layer). The bonded wafer, on the other hand, is an SOI substrate obtained by: bonding two single crystal silicon wafers, with an oxide layer in between; and then turning one of the two wafers into a thin film.
A Metal-Oxide-Semiconductor Field Effect Transistor (“MOSFET”) formed in the SOI layer of such an SOI substrate has high radiation resistance and high latch-up resistance, shows high reliability, suppresses a short channel effect which tends to occur as a device size becomes smaller, and enables an operation with low power consumption, as well. In addition, because the operating region of a device is isolated from the substrate itself in terms of dielectric capacitance, signal transmission speed is enhanced, and thus high-speed operation of the device is realized. For those reasons, the SOI substrate has been the most promising candidate of a high performance semiconductor substrate for MOS-LSIs of the next generation.
Among the variety of the SOI substrates, the SIMOX wafer has an advantage in its excellent uniformity of the thickness of the SOI layer. In the SIMOX wafer, it is possible to form an SOI layer 0.4 μm or less in thickness and even to control the thickness of the SOI layer to 0.1 μm, or even smaller. An SOI layer 0.1 μm or less in thickness, for example, can be used for forming a MOS-LSI for fully depletion type operation. In this case, as the thickness of the SOI layer itself is in a proportional relationship with the threshold voltage of a MOSFET, the uniformity of the thickness of the SOI layer constitutes an important quality aspect for producing devices of uniform performance at a high yield. In this regard, the SIMOX wafer having an SOI layer excellent in uniformity of the thickness has been the most promising candidate of the substrates for MOSFETs of the next generation.
A MOS-LSI formed on an SOI substrate is capable of realizing the excellent properties described above, such as the enhanced radiation resistance and latch-up resistance, the operation with low power consumption, the ultra-high-speed operation, etc., due the device active area being dielectrically isolated from the substrate itself by laying the BOX layer which is an insulator between them. In this respect, the BOX layer should have as perfect an insulating property as possible. More particularly, it is needed that the density of leak defects in the BOX layer (hereinafter called pinhole defects) are as low as possible and the dielectric breakdown property is as close to that of a thermally-grown oxide layer as possible.
In the production of a SIMOX wafer, oxygen ions are implanted by applying, usually, a fixed amount of accelerating energy, typically, an accelerating voltage of 200 kV or so. It is widely known that, in the SIMOX structure formed after a high temperature heat treatment, a continuous, uniform and high quality BOX layer can be obtained only when the implantation dosing amount of the oxygen ions is 1.5×1018 cm−2 or more, or within a limited range from 2.5 to 4.5×1017 cm−2 (refer to S. Nakashima and K. Izumi, the Journal of Materials Research, Vol. 8, p. 523 (1993), for example). The SIMOX wafer produced by implanting the oxygen ion amount specified above is conventionally called: a high dose SIMOX substrate when produced by implanting the oxygen ion amount in the former range; or a low dose SIMOX substrate when produced by implanting the oxygen ion amount in the latter range.
The high dose SIMOX substrate and the low dose SIMOX substrate have respective characteristics, and they are used for different applications according to the characteristics. Out of the two types of SIMOX substrates, in the case of the low dose SIMOX substrate, the dosing amount of oxygen ion implantation is comparatively small, therefore the threading dislocation density of the SOI layer is decreased, and the low dose SIMOX substrate is expected to be a technology capable of realizing low-cost production. The low dose SIMOX substrate, however, has problems such as the relatively large density of pinhole defects in the BOX layer, the high probability of dielectric breakdown property of the BOX layer becoming insufficient and so forth, because the BOX layer is thin. With regard to the BOX quality of the low dose SIMOX substrate, when it is attempted to increase the thickness of the BOX layer by simply increasing the oxygen ion implantation amount, although the pinhole defects decrease, granular substances consisted of silicon (hereinafter referred to silicon islands) are generated in quantities inside the BOX layer and, as a result, the breakdown electric field of the BOX layer is lowered. However, it has been pointed out that, when the oxygen ion implantation dosing amount is decreased, in contrast, though the silicon islands decrease and the breakdown electric field of the BOX layer is improved, the density of the pinhole defects increases as the oxygen ion implantation dosing amount decreases. It has therefore been very difficult to simultaneously improve the whole quality of the BOX layer in the low dose SIMOX substrate produced according to a conventional technology.
As a solution to contribute to the quality improvement of the BOX layer of the low dose SIMOX substrate, a technology has been proposed which makes use of internal oxidation at a high temperature (internal thermal oxidation process, hereinafter called the ITOX technology in abbreviation) (see Japanese Patent Publication No. H07-263538, or S. Nakashima et al., The Journal of Electrochemical Society, Vol. 143, p. 244). According to the ITOX technology, a thermally-grown oxide layer grows on the substrate surface through an oxidation treatment at a high temperature, and, at the same time, some amount of thermally-grown oxide layer at the upper interface of the BOX layer, and thus it becomes possible to make the BOX layer thicker. Further, it has been reported that the pinhole defects are decreased as a result, and the breakdown electric field is improved at the same time. In the ITOX technology, however, it is most likely for the surface oxide layer to grow by not less than 10 times the increase in the thickness of the BOX layer. For this reason, to secure a prescribed thickness of the SOI layer in the SIMOX structure finally obtained, it may be necessary to restrict the amount of oxidation of the substrate surface. As a consequence, there has been a natural limit in the increment of the BOX layer thickness.
As a measure to make the BOX layer of a SIMOX substrate thicker without being influenced by the above restriction, it has been proposed that, in a method wherein a series of oxygen ion implantations is conducted while changing the average depth of the implantation step-by-step or continuously and a heat treatment at a high temperature is applied thereafter, the cumulative distribution of implanted oxygen is controlled so that it falls, after the series of oxygen ion implantation, within a range in which the silicon islands are not generated, and that the distribution has a single peak so as to form a single BOX layer after the high temperature heat treatment (see Japanese Patent Publication No. H7-201975). With this technique, it is theoretically possible to obtain a high quality BOX layer even under dosing conditions outside the so-called dose window, under which dosing conditions it is impossible to obtain a good quality BOX layer when the oxygen ion is implanted using a fixed accelerating voltage. However, it is necessary, in the proposed technique, to control the peak value of the distribution of implanted oxygen to not more than 2.25×1022 cm−3, i.e. about a half of 4.48×1022 cm−3, which is the oxygen concentration in silicon dioxides, in order to prevent the silicon islands from forming in the BOX layer, and, if a plurality of peaks appear in the distribution of implanted oxygen after the series of oxygen ion implantation caused by changes in process conditions or other factors, precipitation takes place during the high temperature heat treatment around each of the peaks and a single BOX layer is not formed. Because of the unstable nature of the proposed method as described above, it has a problem that precise and delicate control of the process is required in order to avoid the instability.
In the meantime, as a possible solution in which oxygen ion implantation and high temperature heat treatment are repeated, a technology has been proposed wherein oxygen ion implantation at a fixed accelerating energy and high temperature heat treatment are repeated for the purpose of improving the quality of a high dose SIMOX substrate (see Japanese Unexamined Patent Publication No. H1-17444). The object of the proposed technology is to reduce defects, such as threading dislocations, in the SOI layer finally obtained by reducing the damage generated in the substrate at each step of the implantation through dividing the implantation of a required dosing amount into a plurality of steps and recovering further from the reduced damage through applying a high temperature heat treatment after each of the implantation steps. In this technique, however, the implantation of the oxygen ion amount conventionally employed for producing a high dose SIMOX substrate is divided into several steps, and it causes no change in the thickness of the BOX layer finally obtained. Besides, it has been pointed out that, using the technique, waviness occurs conspicuously in the BOX layer formed at an intermediary step and, as a consequence, the flatness of the interface of the BOX layer finally obtained is deteriorated.
As a measure to avoid the problem, a proposal has been made, wherein the implantation is divided into two steps and a lower accelerating energy than that in the first step implantation is applied in the second step implantation, and by so doing, oxygen ions are implanted preponderantly at the interface of the BOX layer formed after the first step heat treatment and the SOI layer, and thus the flatness of the interface is improved (see Japanese Patent Publication No. H4-249323). However, this technology is also an invention aiming at improving the quality of the high dose SIMOX substrate and, as the implantation of the total oxygen ion amount conventionally employed for producing a high dose SIMOX substrate is divided into steps, like in the previous example, it causes no change in the thickness of the BOX layer finally obtained. In addition, because the oxygen ions are implanted in the second step on the upper side of the BOX layer already formed in the first step, the SOI layer finally obtained tends to be thin. The proposed technology is, therefore, not suitable for increasing the thickness of the BOX layer while maintaining the thickness of the SOI layer.
The entire disclosures of all publications referred to herein are incorporated by reference in their entireties.